GtkCellRendererAccel: Accept modifiers short cut keys

Control-Shift, Shift-Alt can be received on the cell.

GNOME/gtk#1128
4 jobs for gtk-4-accel in 20 minutes (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Build
passed #49820
fedora-x86_64

00:07:44

passed #49821
win32
msys2-mingw32

00:07:49

 
  Flatpak
passed #49822
flatpak:demo

00:12:11

passed #49823
flatpak:widget-factory

00:09:43